What is the default order of a list returned from a Django filter call?

Question

Short Question
What is the default order of a list returned from a Django filter call when connected to a PostgreSQL database?

Background
By my own admission, I had made a poor assumption at the application layer in that the order in which a list is returned will be constant, that is without using ‘order_by’. The list of items I was querying is not in alphabetic order or any other deliberate order. It was thought to remain in the same order as which they were added to the database.

This assumption held true for hundreds of queries, but a failure was reported by my application when the order changed unknowingly. To my knowledge, none of these records were touched during this time as I am the only person who maintains the DB. To add to the confusion, when running the Django app on Mac OS X, it still worked as expected, but on Win XP, it changed the order. (Note that the mentioned hundreds of queries was on Win XP).

Any insight to this would be helpful as I could not find anything in the Django or PostgreSQL documentation that explained the differences in operating systems.

Example Call

required_tests = Card_Test.objects.using(get_database()).filter(name__icontains=key)

EDIT
After speaking with some colleague’s of mine today, I had come up with the same answer as Björn Lindqvist.

Looking back, I definitely understand why this is done wrong so often. One of the benefits to using an ORM Django, sqlalchemy, or whatever is that you can write commands without having to know or understand (in detail) the database it’s connected to. Admittedly I happen to have been one of these users. However on the flip-side of this is that without knowing the database in detail debugging errors like this are quite troublesome and potentially catastrophic.

Asked By: Adam Lewis

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Source

Answer 1

There is NO DEFAULT ORDER, a point that can not be emphasized enough because everyone does it wrong.

A table in a database is not an ordinary html table, it is an unordered set of tuples. It often surprises programmers only used to MySQL because in that particular database the order of the rows are often predictable due to it not taking advantage of some advanced optimization techniques. For example, it is not possible to know which rows will be returned, or their order in any of the following queries:

select * from table limit 10
select * from table limit 10 offset 10
select * from table order by x limit 10

In the last query, the order is only predictable if all values in column x are unique. The RDBMS is free to returns any rows in any order it pleases as long as it satisfies the conditions of the select statement.

Though you may add a default ordering on the Django level, which causes it to add an order by clause to every non-ordered query:

class Table(models.Model):
    ...
    class Meta:
        ordering = ['name']

Note that it may be a performance drag, if for some reason you don’t need ordered rows.

Answered By: Björn Lindqvist

Answer 2

If you want to have them returned in the order they were inserted:

Add the following to your model:

created = models.DateTimeField(auto_now_add=True, db_index=True)
# last_modified = models.DateTimeField(auto_now=True, db_index=True)

class Meta:
    ordering = ['created',]
    # ordering = ['-last_modified']  # sort last modified first

Answered By: Risadinha

What is the default order of a list returned from a Django filter call?

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